Integrated nutplate and clip for a floating fastener and method of manufacture and assembly

ABSTRACT

An inexpensive and reliable floating fastener is provided by manufacturing an integrated nutplate into the backside of the interior structural member. A retention clip engages the nutplate to capture the nut while allowing the nut to float. The integrated nutplate roughly aligns the floating nut to the axial through-hole in the structural member and provides the torque resistance required to drive the screw into the nut. The retention clip holds the nut in place and provides the axial resistance required for the lead chamfer of the screw to engage the nut and resist the axial loading on the screw during installation. The nutplate is designed to facilitate cost-effective manufacturing. The per hole cost of the integrated floating-fastener is approximately 30% of the cost of the industry standard riveted floating fastener.

GOVERNMENTAL LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contract No.N00019-03-C-0001 awarded by Naval Air Systems Command.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to floating fasteners and, more particularly, tothe design, manufacture and assembly of a floating fastener having anintegrally-formed nutplate and retention clip that provides the samereliable fastener installation at far lower cost.

2. Description of the Related Art

In many structural applications, structural members need to be fastenedtogether. Oftentimes the structural members are too thin, too soft orotherwise too fragile to simply drive a screw through the members toform a reliable joint. Furthermore, misalignment of the structuralmembers will exert a side loading on the screw that will limit thestrength of joint. A common solution is to form aligned axialthrough-holes in the structural members having a diameter greater thanthat of the screw threads. A threaded nut is held on one side and thescrew is driven through the axial through-holes into the nut so that thescrew is placed under tension with no side loading to form a strong andreliable bolted joint at the interface of the two members.

To support cost-effective manufacturing and assembly, the axialthrough-hole on the interior structural member is oversized, whichrelaxes the positional tolerance on manufacturing the holes andassembling the structural device to align the axial through-holes. Inmany applications, there may be dozens of through-hole pairs that needto be simultaneously aligned and then fastened. To further complicatematters, in situations referred to as ‘blind access’ the machine ortechnician that is installing the screw does not have access to thebackside of the assembly to hold the nut. In these cases, a ‘floatingfastener’ is pre-assembled on the backside of the interior structuralmember. The floating fastener includes the threaded nut and a nutplatethat captures the nut but allows it to ‘float’ i.e. move around freelyinside the nutplate, to accommodate misalignment of the axialthrough-holes within a designed for tolerance. The lead chamfer on thescrew will engage the nut and move it over so that the nut and screw areproperly aligned.

Due in large part to the inability to access the backside of thestructure once assembly has begun, the floating fastener assemblies mustbe highly reliable; they must work every time. Rework is slow andexpensive. The floating fastener must have a low risk of installationdamage e.g., damage to the structural members and particularly the axialthrough-holes, and must have a low risk to installed performance e.g.the nut won't fall off prior to assembly and the nutplate will providethe requisite axial and torque resistance to hold the nut in place toinstall the screw properly. Without sacrificing reliability, the “perhole” cost of each fastener including components and labor should be aslow as possible. Structural applications may require dozens of floatingfasteners and the costs add up quickly.

As illustrated in FIG. 1, existing designs of a floating fastener 10that are currently available including “riveted”, which is the MIL-specstandard, “click bond” and “press fit” place a nut 12 (e.g. MIL-spec NAS1794) including a threaded barrel 14 on a base 16 in a discrete nutplate18. The blank nutplate 18 is machined with four protrusions 20 at itscorners that are crimped to capture the base 16 of nut 12 yet allow itto float within the cage. The floating fastener is secured by some‘means’ to the backside of an interior structural member 22 so that thebarrel lies within the extent of an oversized axial through-hole 24. Anexterior structural member 26 having axial through-hole 28 is placedover the interior structural member 22 so that their through-holes 28and 24 are roughly aligned providing enough overlap to insert a screw 30so that its lead chamfer 32 will engage the barrel and move the nut overso that the nut and screw are properly aligned. The cost associated withmachining of the nutplate 18 and the labor to crimp the nutplate tocapture the nut is not insignificant, approximately $0.90 to $2.40 perhole plus labor. All of the existing designs use some singular means' toalign the nutplate and nut to the axial through-hole and hold it inplace, to provide the axial resistance required so that the lead chamferengages the nut and resists the axial load from driving the screw intothe nut, and to provide the torque resistance required to prevent thenut and nutplate from turning. The different means vary in reliabilityand cost.

As shown in FIG. 2 a, a riveted floating-fastener 40 includes a nut 42and nutplate 44 in which the nutplate is machined with a pair of flanges46 on opposite sides of the nut each having a precision hole 48 formedtherein for receiving a rivet. In an alternate embodiment, for use incorners for example, both precision holes 48 are formed in a singleflange to one side of the nut. The precision holes 48 must be alignedand spaced to precisely match complementary holes formed on oppositesides of the axial through-hole in the interior structural member.Rivets are driven through precision holes 48 into the mating holes inthe interior member. Each rivet is a compressed metal column thatexpands outward to fill the hole in the structural member to hold thenutplate in place. The riveted floating-fastener is the mil-specstandard because of the low technical risk associated with the rivets,they provide a very strong and reliable joint to hold the nutplate.There is some risk of installation damage during rivet installation. Thetradeoff is that the assembled cost is quite high, approximately $6.69per hole for Raytheon's JSOW. The riveted fastener requires fivemachining operations using three different tools and three differentassembly operations.

As shown in FIG. 2 b, a click-bond floating fastener 50 includes a nut52 and a nutplate 54 that is adhesively bonded to the backside of theinterior structural member to position nut 52 below the axialthrough-hole in the member. An applicator 56 is inserted through the nutand used to align the nut and nutplate to the axial through-hole andbond the nutplate using a two-part adhesive and is then removed. Pushingon the applicator during assembly poses some risk. This approachrequires only a single machining operation and a single assemblyoperation but uses two-part adhesives that require 7 days to cure. Thisfastener uses the same mil-spec nut and a modified nutplate withoutflanges. The total cost is lower than the riveted fastener,approximately $4.08 per hole, but still high. However the adhesive bondis not as reliable as the rivets. The bond is known to fail occasionallyunder the axial and/or torque loading when installing the screw. Thebond can also be stressed through a difference in thermal expansion attemperature extremes.

As shown in FIG. 2 c, a press-fit floating fastener 60 includes a nut62, a sleeve 64 formed with teeth around is circumference and a nutplate66. The sleeve is mounted on a mandrel 68 that pulls the sleeve throughthe axial through-hole in the interior structural member. The teethdeform the metal on the inside of the hole to hold the nutplate in placeand provide axial and torque resistance. Once press-fit into the holethe mandrel is removed. This approach requires only a single machiningoperation and a single assembly operation using a special tool. Thetotal cost is similar to the adhesive bonding, approximately $4.12 perhole. The installation risk is also similar to bonding in that pressingthe teeth into the hole may cause damage. The bigger problem is that thedeformation of the axial through-hole creates ‘stress risers’ thatweaken the interior structural member and reduce the reliability of thejoint.

The industry has an unfulfilled need for a ‘floating-fastener’ thatprovides the same reliability as the riveted fastener but at a muchlower total cost per hole. Preferably any such solution could use theMIL-spec nut currently accepted by the industry.

SUMMARY OF THE INVENTION

The present invention provides an inexpensive and reliable floatingfastener.

This is accomplished by manufacturing an integrated nutplate into thebackside of the interior structural member and providing a retentionclip that engages the nutplate to capture the nut while allowing the nutto float. The integrated nutplate roughly aligns the floating nut to theaxial through-hole in the structural member and provides the torqueresistance required to drive the screw into the nut. The retention clipholds the nut in place and provides the axial resistance required forthe lead chamfer of the screw to engage the nut and resist the axialloading on the screw during installation. The nut can be the samecommercially available nut as used in conventional floating nutplatedesigns.

Milling this type of structure in the backside of an otherwise smoothinterior structural member would appear to be complicated and thusexpensive. However, high speed multi-axis milling machines allow theintegrated nutplate to be milled virtually free, assuming a good designis selected. A ‘good’ nutplate design is one that provides the requiredtorque resistance, facilitates the use of a simple clip to capture thenut and is efficient to mill. These requirements dictate that thenutplate include first and second discrete linear members each having atleast one through-hole that is substantially perpendicular to the axialthrough-hole in the structural member. In an embodiment, the first andsecond discrete linear members are a pair of parallel rails milled onopposite sides of the axial through-hole. In many applications, the samepair of parallel rails can be used for multiple axial through-holesaligned in a linear configuration. The discrete linear members, e.g.parallel rails, lie along or parallel to an axis of the milling machineso that their formation does not slow the milling of the interiorstructural member.

In an alternate embodiment, the integrated nutplate is first molded aspart of the structural member and then milled to finish the part.Milling is required in order to achieve the required precision of thediscrete linear members. The design criteria for a good nutplate are thesame.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, as described above, is a diagram of a blind-access floatingfastener;

FIGS. 2 a-2 c, as described above, are diagrams of riveted, bonded andpress-fit embodiments of the floating fastener;

FIGS. 3 a-3 c are perspective, side and bottom views of a floatingfastener including an integrated nutplate and clip in accordance withthe present invention.

FIG. 4 is a side view of a screw and floating fastener assembly;

FIG. 5 is a diagram of the axial resistance provided by the clip inresponse to the axial load placed on the screw during installation toallow the screw to engage the nut;

FIGS. 6 a and 6 b are diagrams of the torque resistance provided by theintegrated nutplate in response to the torque placed on the screw duringinstallation to allow the screw to be threaded through the nut;

FIGS. 7 a and 7 b are a diagram of a portion of missile hull providedwith an integrated nutplate that spans multiple through-holes forfastening with a portion of a complementary shell and a close-up of theintegrated nutplate;

FIG. 8 is a diagram of a 5-axis milling machine for fabricating astructural member having an integrated nutplate for one or morethrough-holes;

FIGS. 9 a-9 f illustrate the fabrication of the integrated nutplatedepicted in FIGS. 3 a-3 c;

FIG. 10 is a table of estimated assembled cost for the known riveted,click bond and press fit fasteners and the integrated fastener for anexemplary missile configuration in accordance with the presentinvention; and

FIG. 11 is an alternate embodiment of the integrated nutplate andretention clip in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an inexpensive and reliable floatingfastener by manufacturing an integrated nutplate into the backside ofthe interior structural member. A retention clip engages the nutplate tocapture the nut while allowing the nut to float. The integrated nutplateroughly aligns the floating nut to the axial through-hole in thestructural member and provides the torque resistance required to drivethe screw into the nut. The retention clip holds the nut in place andprovides the axial resistance required for the lead chamfer of the screwto engage the nut and resist the axial loading on the screw duringinstallation. The nutplate is designed to facilitate cost-effectivemanufacturing. For Raytheon's JSOW the per hole cost is reduced from$6.75 for the MIL-spec standard riveted floating fastener toapproximately $1.21. The JSOW includes 160 floating fasteners for asavings of approximately $886 per weapon. The exact per hole cost andtotal savings will vary depending upon the application but these numbersare representative of the commercial benefit provided by the integralfloating fastener without sacrificing reliability.

An integrated floating fastener 100 prior to final assembly isillustrated in FIGS. 3 a through 3 b. Fastener 100 includes anintegrated nutplate 102 that has been integrally formed on the backsideof an interior structural member 104. In this embodiment, nutplate 102comprises a pair of parallel rails 106 and 108 suitably spaced at equaldistances on other side of oversized axial through-hole 110. Each railsuitably includes a pair of through-holes 112, 114 and 116, 118 that areapproximately perpendicular to the axial through-hole. Although otherconfigurations of at least two discrete, linear rails are possible, thepair of parallel rails provides a number of manufacturing, assembly andperformance benefits.

Fastener 100 includes a nut 120 such as MIL-spec NAS 1794 having athreaded barrel 122 on a base 124 that is placed between rails 106 and108 and roughly aligned with axial through-hole 110. A U-shapedretention clip 126 is inserted through through holes 112, 114 in rail106, around the threaded barrel 122 and over base 124 and extendingthrough the respective pair of through-holes 116, 118 in opposing rail108 so that each said nut 120 is captured by the parallel rails and theretention clip but allowed to float. The clip is suitably ‘pinched’ atits midsection so that it will not fall out once in place. The railsmust be spaced far enough apart to let the nut float by a designedtolerance but close enough that the barrel 122 at least partiallyoverlaps axial through-hole 110 and that when rotated base 124 engagesrails 106, 108.

Final assembly of the integrated floating fastener 100 and the axial andtorque resistance provided by the fastener are illustrated in FIGS. 4-6.An exterior structural member 130 is positioned over interior structuralmember 104 so that its axial through-hole 132 is approximately alignedto oversized through-hole 110. A screw 134 is driven through said axialthrough-holes 132 and 110 so that a lead chamfer 136 on the screw alignsthe nut's threaded barrel 122 and the screw. Nut base 124 engagesretention clip 126 producing axial forces 138 that resist the axial load140 placed on the screw during installation. The strength of theretention clip, typically dictated by the diameter of the wire and thespacing between the rails, must be strong enough to provide the requiredresistance. Furthermore, the parallel rail/U-clip configuration producesaxial resistance along both arms of the clip between the rails. Thisdistributed and symmetric force keeps the nut from deflecting at anangle as the lead chamfer engages the barrel. Nut base 124 also engagesrails 106 and 108 producing rotational forces 142 that resist the torque144 placed on the screw during installation. As the screw is driven intothe barrel, the applied torque 144 will cause the nut to rotateinitially engaging one of the two rails 106 or 108. This will produce alateral force 142 that will cause the screw to realign slightly untilthe base engages both rails 106 and 108 before the screw contacts theinside of either axial through-hole. The torque resistance is providedat only two points in a plane 146 substantially perpendicular to theaxis 148 of the screw. As a result, no side-loading from either axialthrough-hole is applied to the screw so that the fastened screw is innear perfect tension 150 as desired.

The final assembly provides a joint at the interface of the interior andexterior structural members that is as reliable as the riveted floatingfastener without the installation risk associated with riveting thediscrete nutplate to the structural member. The final assembly is donewithout rivets, adhesive or deformation of the axial through-holes in apress fit configuration. Furthermore, the final assembly is a fractionof the cost of any of the known discrete floating-fasteners.

As mentioned previously, typical applications may use manyfloating-fasteners to reliably fasten one structural member to another.Raytheon's JSOW is one such example. A U-shaped extended hull 200 isprovided to carry the explosives. A complementary shell 202 ispositioned over the hull and fastened in 80 different places usingfloating-fasteners. Only representative portions of the hull and shellare shown. In this, and many other applications, subsets of multipleaxial through-holes will lie along an axis in a linear configuration. InJSOW, there are lines on both sides of the hull and lines both ends ofthe hull (the lines on the ends are actually U-shaped to conform to thehull). The sides may have 21 holes per line and the ends 19 per line.Using conventional approaches, a separate floating-fastener albeitriveted, bonded or press-fit would have to be individually assembled foreach hold.

As illustrated in FIGS. 7 a and 7 b, the integrated nutplate and moreparticularly the parallel rail configuration of the integrated nutplateallow for considerable savings in final assembled cost. As shown onepair of rails 208 is formed into the hull 200 on opposite sides of thelinear arrangement of axial through-holes 210 for each subset. In theJSOW weapon only four rail pairs are required for the 80 holes; one pairfor each side and for each end. To complete preassembly of the interiorstructural member, a technician simply inserts the nut 212 and slidesthe U-shaped retention clip 214 through the rails to capture the nut andquickly repeats the process. No special tools or applicators arerequired to complete preassembly.

For the integrated floating-fastener to be as reliable as the MIL-specriveted fastener and to achieve significant cost savings, the nutplatedesign must be efficient to manufacture and assemble. Structural membersare typically either milled or molded to form the smooth shapestypically encountered depending on the materials or application. Millingis typically used with materials such as aluminum, titanium or becausethe raw material stock is typically milled to a final shape. Molding istypically used with plastics or composite materials because rawmaterials are typically cured to a solid finished form to reach thefinal shape.

Milling this type of structure in the backside of an otherwise smoothinterior structural member would appear to be complicated and thusexpensive. Currently, machining on a 5-axis high speed machine costsabout $400 per hour. If the milling of the integrated nutplate slows theprocess to any appreciable degree the additional milling costs willoutweigh the component and assembly savings. However, high speedmulti-axis milling machines allow the integrated nutplate to be milledvirtually free, assuming a good design is selected. As described above‘good’ nutplate design is one that provides the required torqueresistance and facilitates the use of a simple clip to capture the nut,and is efficient to mill. These requirements dictate that the nutplateinclude first and second discrete linear members each having at leastone through-hole that is substantially perpendicular to the axialthrough-hole in the structural member. If the members are non-linear orconnected, the control of the servo motors becomes more complex whichslows the milling process and time is money. In an embodiment, the firstand second discrete linear members are a pair of parallel rails milledon opposite sides of the axial through-hole. In many applications, thesame pair of parallel rails can be used for multiple axial through-holesaligned in a linear configuration. The discrete linear members, e.g.parallel rails, lie along or parallel to an axis of the milling machineso that their formation does not slow the milling of the interiorstructural member.

FIGS. 8 and 9 a-9 f provide a simplified depiction of a 5-axis millingmachine 300 and the steps for milling a pair of rails 302 and 304 toform an integrated nutplate on the back side of a structural member 306.Milling machine 300 includes a table 308 that supports a block ofmaterial 310 and a rotating bit 312 to remove material from the block toform the structural member. The table is actuated by linear servo motors314 and 316 to move along the X and Y axis respectively and by motors318 and 320 to rotate around the X and Y axis in the polar and azimuthalangles respectively. A linear servo motor 322 moves the rotating bitback-and-forth along the Z-axis. The specific design 324 of a structuralmember is loaded into a controller 326 that controls all of thedifferent motors to mill the block of material 310 to form theintegrated nutplate on the back side of the structural member.

In general, milling is very efficient to form simple shapes. If thecontroller can step either the X or Y axis servo motor and then make along cut down the other of the Y or X axis the material can be removedand the structure formed quickly. Conversely, if the machine has to makeshort cuts or be controlled to form curves or angles than the processslows dramatically. The machine has to calculate and move in a 3-Dcutting path to provide the curves. In addition, the machine may have tomake a lot of passes to get the shape smooth.

As shown in FIGS. 9 a-9 e, the pair of rails 302 and 304 for a single ormultiple through-holes that define the integrated nutplate can be milledinto the backside of structural member 306 virtually for free. The onlyvariations in the process are: (a) when the bit gets to the depth of therails and the linear servo motor along the X-axis is stepped to theposition of the rails, the bit is pulled back so that the material isnot removed and step past the rail and (b) once the rails are formed,the table is rotated 90 degrees around the Y-axis and the bit actuatedto form the through-holes in the rail. More specifically, the block ofmaterial 310 is milled to reduce its thickness to approximately thedesired thickness. Note, in this example the structural member is flatbut could be a curved shape as in the JSOW hull, this step would be usedto form that basic shape in the block. Once at the proper depth, thecontroller actuates the linear servo motor 314 so that the relativemotion of the bit moves left-to-right along the X-axis removing materialuntil the position of the first rail is reached (FIG. 9 c). The bit ispulled back and the linear servo member 314 actuated to step the bit tothe other side of the rail where it is lowered. The other linear servomember 316 than moves the table so that the bit makes a long cut downthe Y-axis to define the first rail 302 (FIG. 9 d). One or more steps ofthe X-axis servo motor 314 may be required to remove the materialbetween the rails. The bit is pulled back and the linear servo member314 actuated to step the bit to the other side of the second rail 304where it is lowered. The other linear servo member 316 than moves thetable so that the bit makes a long cut down the Y-axis (FIG. 9 e) todefine the second rail 304. The X-axis servo motor is then stepped andlong cuts made to finish the member (FIG. 9 f). Motor 320 rotates thetable 90 degrees so that the bit can mill the through-holes in rails 302and 304.

In an alternate embodiment, the integrated nutplate is first molded aspart of the structural member and then milled to finish the part.Milling is required in order to achieve the required precision of thediscrete linear members. The design criteria for a good nutplate are thesame. If a large number of the same part are required, molding is anoption. Formation of the mold, usually out of steel, is very expensive.For metallic finished parts such as aluminum or titanium, the parts areeither forged by heating the material and pressing it into the mold orcast by pouring molten metal into the mold. Non-metallic parts such ascarbon fibers, fiberglass and other exotic materials can be compressionmolded. The materials are laid into the mold and cured in a liquidresin. In almost all cases, the molded parts still require precisionmilling to achieve the size, shape and smoothness tolerances required.

A cost comparison for the riveted, bonded, press-fit and integratedfloating-fasteners for Raytheon's JSOW is depicted in FIG. 10. The perhole costs are approximately: riveted $6.69, bonded $4.08, press-fit$4.12 and integrated $2.06. Even using conservative numbers foradditional milling time (30 minutes total @$400/hr) and assembly (15minutes @$100/hr), the per hole cost is reduced substantially to lessthan 31% of the riveted fastener and less than 61% of the bonded orpress-fit fastener. As compared to the riveted fastener, the integratednutplate saves some expense in component cost but the bulk of thesavings is derived from labor to assemble the fasteners. Withoutsacrificing the MIL-spec reliability, integrated nutplate can save over$700 in cost.

An alternate albeit less desirable embodiment of an integrated floatingfastener 400 is depicted in FIG. 11. In this configuration, a pair oflinear discrete rails 402 and 404 are integrally formed on the backsideof structural member 406 perpendicular to one another. First and secondclips 408 and 410 are inserted around the barrel 412 of nut 414 into thethrough-holes in the respective rails 402 and 404, respectively. Theclips will provide the axial resistance required to insert the leadchamfer of the screw into the barrel. The pair of rails will providetorque resistance at two points in the same plane sufficient to resistthe application torque and fasten the screw into the nut. Although thisconfiguration could be used it is less efficient to mill, requires twoclips (or one more complicated clip), and provides a somewhat lessreliable cage to capture the nut so that it does not fall out prior toor during assembly. Other configurations of two or more discrete linearmembers and one or more retention clips that satisfy the design criteriamay be used depending upon the application.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

1. A method of fastening structural members, comprising: providing afirst structural member having a first axial through-hole; providing asecond structural member having a second axial through-hole larger thansaid first axial through-hole and an integrally-formed nutplate on thebackside of the second structural member near the second axialthrough-hole, said nutplate comprising first and second substantiallyparallel rails on opposite sides of the second axial-through hole, eachrail having a pair of through-holes substantially perpendicular to thesecond axial through-hole; positioning a nut including a threaded barreland a base between the first and second rails on the backside of thesecond structural member; inserting a U-shaped retention clip throughsaid pair of through-holes in one said rail around the threaded barreland extended through the pair of through-holes in the other said rail sothat the base is captured by the nutplate and U-shaped retention clipallowing the nut to float; positioning the first structural member overthe front side of the second interior structural member to align thefirst axial through-hole to the larger second axial through-hole; anddriving a screw through said first and second axial through-holes sothat a lead chamfer on the screw aligns the nut's threaded barrel andthe screw, said nut base engaging said clip which resists an axial loadplaced on the screw and engaging said nutplate's first and second railswhich resist a torque placed on the screw so that said screw is fastenedinto said threaded barrel by the applied torque.
 2. The method of claim1, further comprising: milling a block of material to form the secondstructural member with said pair of parallel rails defining theintegrally-formed nutplate.
 3. The method of claim 2, further comprisingusing a 5-axis milling machine to mill the block of material, saidmachine including linear servo motors to effectuate milling along the Xand Y axis, said parallel rails lying parallel to said X or said Y axis.4. A method of fastening structural members, comprising: providing afirst structural member having first axial through-holes in a lineararrangement; providing a second structural member having a plurality ofsecond axial through-holes larger than said first through-holes in acomplementary linear arrangement and an integrally-formed nutplatecomprising first and second parallel rails on opposite sides of thecomplementary linear arrangement of second axial through-holes on thebackside of the second structural member, each rail having a pair ofthrough-holes adjacent each said second axial through-hole; positioninga plurality of nuts each including a threaded barrel and a base betweenthe first and second parallel rails on the backside of the secondstructural member over the respective second through-holes; inserting aplurality of U-shaped retention clips through respective pairs ofthrough-holes in one rail, around the threaded barrel and extendingthrough the respective pair of through-holes in the opposing rail sothat each said nut is captured by the parallel rails and the respectiveclips but allowed to float; positioning the first structural member overthe front side of the second structural member to align the first axialthrough-holes to the larger second axial through-holes; and drivingrespective screws through said first and second axial through-holes sothat a lead chamfer on the screw aligns the nut's threaded barrel andthe screw, said nut base engaging said clip which resists an axial loadplaced on the screw and engaging said nutplate's first and second railswhich resist a torque placed on the screw so that said screw is fastenedinto said threaded barrel by the applied torque.